Solar cell and method for manufacturing the same

ABSTRACT

A method for manufacturing a solar cell is discussed. The method may include injecting first impurity ions at a first surface of a substrate by using a first ion implantation method to form a first impurity region, the substrate having a first conductivity type and the first impurity ions having a second conductivity type, and the first impurity region having the second conductivity type; heating the substrate with the first impurity region to activate the first impurity region to form an emitter region from the first impurity region; etching the emitter region from a surface of the emitter region to a predetermined depth to form an emitter part from the emitter region; and forming a first electrode on the emitter part to connect to the emitter part and a second electrode on a second surface of the substrate to connect to the second surface of the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of co-pending U.S. application Ser. No.13/331,787 filed Dec. 20, 2011, which claims priority to Korean PatentApplication No. 10-2010-0131823, filed in the Republic of Korea on Dec.21, 2010, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relate to a solar cell and a method formanufacturing the same.

2. Discussion of the Related Art

Recently, as existing energy sources such as petroleum and coal areexpected to be depleted, interests in alternative energy sources forreplacing the existing energy sources are increasing. Among thealternative energy sources, solar cells for generating electric energyfrom solar energy have been particularly spotlighted.

A solar cell generally includes semiconductor parts that have differentconductivity types, such as a p-type and an n-type, and form a p-njunction, and electrodes respectively connected to the semiconductorparts of the different conductivity types.

When light is incident on the solar cell, electron-hole pairs aregenerated in the semiconductor parts. The electrons move to the n-typesemiconductor part and the holes move to the p-type semiconductor part,and then the electrons and holes are collected by the electrodesconnected to the n-type semiconductor part and the p-type semiconductorpart, respectively. The electrodes are connected to each other usingelectric wires to thereby obtain electric power.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a method for manufacturing asolar cell includes injecting first impurity ions at a first surface ofa substrate by using a first ion implantation method to form a firstimpurity region, the substrate having a first conductivity type and thefirst impurity ions having a second conductivity type opposite the firstconductivity type, and the first impurity region having the secondconductivity type, heating the substrate with the first impurity regionto activate the first impurity region to form an emitter region from thefirst impurity region, etching the emitter region from a surface of theemitter region to a predetermined depth to form an emitter part from theemitter region, and forming a first electrode on the emitter part toconnect to the emitter part and a second electrode on a second surfaceof the substrate, which is opposite the first surface of the substrateto connect to the second surface of the substrate.

The heating of the substrate may heat the first impurity portion at 800°C. to 1100° C. in a nitrogen atmosphere.

The etching of the emitter region may remove a portion of the emitterregion from the surface of the emitter region to a depth of 5 nm to 20nm.

The emitter region may be etched by an etchant composed of nitric acidHNO₃, hydrofluoric acid HF and pure wafer.

The heating of the substrate may heat the first impurity portion at 800°C. to 1100° C. in an oxygen atmosphere.

The etching of the emitter region may remove a portion of the emitterregion from the surface of the emitter region to a depth of 20 nm to 35nm.

The emitter region may be etched by an etchant composed of hydrofluoricacid HF and pure wafer.

The emitter part may include a first emitter portion having a firstimpurity doped thickness and a second emitter portion having a secondimpurity doped thickness greater than the first impurity dopedthickness, wherein the etching of the emitter region includes:selectively forming an etch prevention layer on the emitter region toexpose a portion of the emitter region and to cover a remaining portionof the emitter region, and etching the exposed portion of the emitterregion from the surface of the emitter region to the predetermined depthusing the etch prevention layer as a mask, and removing the etchprevention layer, wherein the etched exposed portion of the emitterregion is formed as the first emitter portion and the remaining portionof the emitter region is formed as the second emitter portion.

The first impurity region may include a first impurity portion having afirst impurity doped thickness and a second impurity portion having asecond impurity doped thickness greater than the first impurity dopedthickness, and wherein the injecting of the first impurity ions formsthe first and second impurity portions by use of a mask positioned atthe first surface of the substrate and use of the first ion implantationmethod.

The mask may include a first portion having a first exposing area forforming the first impurity portion and a second portion having a secondexposing area for forming the second impurity portion, the first andsecond exposing areas being areas exposing the substrate in a unit areathereof.

The method may further include forming the first impurity region nothaving the first and second impurity portions at an entire first surfaceof the substrate by injecting the first impurity ions of the secondconductivity type at the entire first surface of the substrate without amask, before forming the first and second impurity portions of the firstimpurity region, wherein the forming of the first and second impurityportions of the first impurity region forms the first and secondimpurity portions by use of the mask positioned at the first impurityregion not having the first and second impurity portions and use of thefirst ion implantation method.

The method may further include injecting second impurity ions at asecond surface of the substrate by using an ion implantation method toform a second impurity region of the first conductivity type, the secondsurface being opposite the first surface of the substrate, heating thesubstrate with the second impurity region to activate the secondimpurity region to form an surface field region from the second impurityregion, and etching the surface field region from a surface of thesurface field region to a predetermined depth to form a surface fieldpart from the surface field region, wherein the second electrode isconnected to the second surface of the substrate through the surfacefield part.

The surface field part may include a first surface field portion havinga first impurity doped thickness and a second surface field portionhaving a second impurity doped thickness greater than the first impuritydoped thickness, wherein the etching of the surface field regionincludes: selectively forming an etch prevention layer on the surfacefield region to expose a portion of the surface field region and tocover a remaining portion of the surface field region, etching theexposed portion of the surface field region from the surface of thesurface field region to the predetermined depth using the etchprevention layer as a mask, and removing the etch prevention layer,wherein the etched exposed portion of the surface field region is formedas the first surface field portion and the remaining portion is formedis formed as the second surface field portion.

The second electrode may be in contact with the second surface fieldportion and is connected to the second surface of the substrate throughthe second surface field portion.

The first and second surfaces of the substrate may be light incidentsurfaces on which light is incident.

The second impurity region may include a first impurity portion having afirst impurity doped thickness and a second impurity portion having asecond impurity doped thickness greater than the first impurity dopedthickness, wherein the injecting of the second impurity ions forms thefirst and second impurity portions by use of a mask positioned at firstsurface of the substrate and use of the second ion implantation method.

The mask may include a first portion having a first exposing area forforming the first impurity portion and a second portion having a secondexposing area for forming the second impurity portion, the first andsecond exposing areas being areas exposing the substrate in a unit areathereof.

The method may further include forming the second impurity region nothaving the first and second impurity portions at an entire secondsurface of the substrate by injecting the second impurity ions of thefirst conductivity type at the entire second surface of the substratewithout a mask, before forming the first and second impurity portions ofthe second impurity region, wherein the forming of the first and secondimpurity portions of the second impurity region forms the first andsecond impurity portions by use of the mask positioned at the secondimpurity part not having the first and second impurity portions and useof the second ion implantation method.

The second electrode may be in contact with the second surface fieldportion and is connected to the second surface of the substrate throughthe second surface field portion.

The first and second surfaces of the substrate may be light incidentsurfaces on which light is incident.

According to another aspect of the invention, a solar cell may include asubstrate into which an impurity of a first conductivity type is doped,an emitter part positioned at a first surface of the substrate, intowhich an impurity of a second conductivity type opposite the firstconductivity type is doped, and comprising a first emitter portionhaving a first impurity doped thickness and a second emitter portionhaving a second impurity doped thickness greater than the first impuritydoped thickness, a first electrode positioned at the second emitterportion and connected to the second emitter portion, and a secondelectrode positioned at a second surface of the substrate and connectedto the substrate, the second surface being opposite the first surface ofthe substrate, wherein a junction surface between the first emitterportion and the substrate is positioned at a same height as a junctionsurface between the second emitter portion and the substrate, and adamage amount existing at the second emitter portion is more than adamage amount existing at the first emitter portion.

The solar cell may further include an anti-reflection layer positionedon the first emitter portion of the emitter part.

The solar cell may further include a surface field part positionedbetween the second surface of the substrate and the second electrode anddoped with an impurity of the first conductivity type.

The surface field part may include a first surface field portion havinga third impurity doped thickness and a second surface field portionhaving a fourth impurity doped thickness greater than the third impuritydoped thickness, a first electrode positioned at the second emitterportion and connected to the second emitter portion, and a damage amountexisting at the second surface field portion is more than a damageamount existing at the first surface field portion.

The second electrode may be positioned on the second surface fieldportion and connected to the second surface field portion.

The first and second surfaces of the substrate may be light incidentsurfaces on which light is incident.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate example embodiments of theinvention and together with the description serve to explain theprinciples of the invention. In the drawings:

FIG. 1 is a partial perspective view of a solar cell according to anexample embodiment of the invention;

FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1;

FIGS. 3A to 3G are cross-sectional views sequentially illustrating amethod for manufacturing a solar cell according to an example embodimentof the invention;

FIG. 4 shows graphs illustrating impurity doped concentrations dependingon changes in depths of an emitter region and an emitter part accordingto an example embodiment of the invention;

FIG. 5 depicts a damage portion existing at an emitter part in a solarcell according to an comparative example;

FIG. 6 is a partial perspective view of a solar cell according toanother example embodiment of the invention;

FIG. 7 is a cross-sectional view taken along line VII-VII of FIG. 6;

FIGS. 8A to 8D are cross-sectional views illustrating portions ofprocesses for manufacturing the solar cell shown in FIGS. 6 and 7;

FIG. 9 is a partial cross-sectional view of a solar cell according toyet another example embodiment of the invention;

FIGS. 10A to 10C are cross-sectional views illustrating portions ofprocesses for manufacturing the solar cell shown in FIG. 9;

FIG. 11 is a partial cross-sectional view of a solar cell according toyet another example embodiment of the invention; and

FIG. 12 shows graphs illustrating an external quantum efficiency and aninternal quantum efficiency of the solar cell shown in FIG. 11 dependingon a wavelength variation of light.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described more fully hereinafter with reference tothe accompanying drawings, in which example embodiments of theinventions are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to the exampleembodiments set forth herein.

In the drawings, the thickness of layers, films, panels, regions, etc.,are exaggerated for clarity. Like reference numerals designate likeelements throughout the specification. It will be understood that whenan element such as a layer, film, region, or substrate is referred to asbeing “on” another element, it can be directly on the other element orintervening elements may also be present. In contrast, when an elementis referred to as being “directly on” another element, there are nointervening elements present. Further, it will be understood that whenan element such as a layer, film, region, or substrate is referred to asbeing “entirely” on another element, it may be on the entire surface ofthe other element and may not be on a portion of an edge of the otherelement.

Reference will now be made in detail to example embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

An example of a solar cell according to an example embodiment of theinvention is described in detail with reference to FIGS. 1 and 2.

As shown in FIGS. 1 and 2, a solar cell 1 according to an exampleembodiment of the invention includes a substrate 110, an emitter part121 positioned at an incident surface (hereinafter, referred to as “afront surface” or “a first surface”) of the substrate 110 on which lightis incident, an anti-reflection layer 130 positioned on the emitter part121, a front electrode part 140 positioned on the emitter part 121, aback surface field part 172 positioned at a surface (hereinafter,referred to as “a back surface” or “a second surface”) opposite thefront surface of the substrate 110, and a back electrode part 150positioned on the back surface of the substrate 110 and the back surfacefield part 172.

The substrate 110 is a semiconductor substrate such as silicon of afirst conductivity type, for example, an n-type, though not required.The semiconductor is a crystalline semiconductor of single crystallinesilicon or poly crystal silicon.

When the substrate 110 is of an n-type, the substrate 110 may be dopedwith impurities of a group V element such as phosphorus (P), arsenic(As), and antimony (Sb). Alternatively, the substrate 110 may be of ap-type. When the substrate 110 is of the p-type, the substrate 110 maybe doped with impurities of a group III element such as boron (B),gallium (Ga), and indium (In).

The front surface of the substrate 110 is textured to form a texturedsurface corresponding to an uneven surface or having unevencharacteristics and including a plurality of projections and a pluralityof depressions. FIG. 1 shows that only an edge of the substrate 110 andonly an edge of the anti-reflection layer 130 on the substrate 110 havea plurality of uneven portions for the sake of brevity. However, theentire front surface of the substrate 110 is the textured surface havingthe plurality of uneven portions, and thus the anti-reflection layer 130on the front surface of the substrate 110 has a textured surface havinga plurality of uneven portions. In an alternative example, the backsurface as well as the front surface of the substrate 110 may have atextured surface.

By the textured surface of the substrate 110 having the plurality ofprojections, the surface area of the substrate 110 and the surface areaof the anti-reflection layer 130 increase and an amount of lightreflected by the substrate 110 decreases and thereby, an amount of lightincident on the substrate 110 increases.

The emitter part 121 is an impurity region obtained by doping thesubstrate 110 with impurities of a second conductivity type (forexample, a p-type) opposite the first conductivity type of the substrate110. Thus, the emitter part 121 forms a p-n junction with the substrate110 (that is, the first conductivity type portion of the substrate 110).

In this example, the emitter part 121 is formed by an ion implantationmethod. Thereby, when compared with an emitter part formed by a thermaldiffusion method, the maximum impurity doped concentration (that is, themaximum surface impurity doped concentration) of a surface (that is, thefront electrode part 140 is positioned thereat and a surface opposite ap-n junction surface between the first conductivity portion of thesubstrate 110 and the emitter part 121) of the emitter part 121increases and an impurity doped thickness of the emitter part 121decreases.

In the thermal diffusion method, for increasing an impurity dopedconcentration of a surface of the emitter part, it is required toincrease a process time, but since the impurity doped thicknessincreases in proportion to the process time, the impurity dopedthickness also increases by the increased impurity doped concentration.

However, when the emitter part 121 is formed by the ion implantationmethod, the maximum surface impurity doped concentration is greater thanthat of the thermal diffusion method, but the impurity doped thicknessof the emitter part 121 decreases.

Thereby, when compared with the emitter part formed by the thermaldiffusion method, the impurity doped thickness (depth) decreases and themaximum surface impurity doped concentration of the emitter part 121increases.

By a built-in potential difference due to the p-n junction of thesubstrate 110 and the emitter part 121, a plurality of electrons and aplurality of holes produced by light incident on the substrate 110 moveto the n-type semiconductor and the p-type semiconductor, respectively.Thus, when the substrate 110 is of the n-type and the emitter part 121is of the p-type, the holes move to the emitter part 121 and theelectrons move to the back surface of the substrate 110.

Because the substrate 110 and the emitter part 121 form the p-njunction, the emitter part 121 may be of the n-type when the substrate110 is of the p-type unlike the example embodiment described above. Inthis instance, the electrons move to the emitter part 121, and the holesmove to the back surface of the substrate 110.

When the emitter part 121 is of the p-type, the emitter part 121 may beformed by doping impurities of a group III element into the substrate110. On the contrary, when the emitter part 121 is of the n-type, theemitter part 121 may be formed by doping impurities of a group V elementinto the substrate 110.

The anti-reflection layer 130 positioned on the emitter part 121 is madeof hydrogenated silicon nitride (SiNx:H), hydrogenated silicon oxide(SiOx:H), or hydrogenated silicon oxy nitride (SiOxNy:H), etc.

The anti-reflection layer 130 reduces reflectance of light incident ontothe substrate 110 and increases selectivity of a specific wavelengthband, thereby increasing the efficiency of the solar cell 1. Further, bythe hydrogen (H) supplied when the anti-reflection layer 130 is formed,the anti-reflection layer 130 performs a passivation function thatconverts a defect, for example, dangling bonds existing on the surfaceof the substrate 110 and around the surface of the substrate 110, intostable bonds to thereby prevent or reduce a recombination and/or adisappearance of charges moving to the front surface of the substrate110 resulting from the defect. Hence, the anti-reflection layer 130reduces loss of charges caused by disappearance of the charges due tothe defect on or around the surface of the substrate 110, to furtherimprove the efficiency of the solar cell 1.

In this example embodiment, the anti-reflection layer 130 has asingle-layered structure, but the anti-reflection layer 130 may have amulti-layered structure such as a double-layered structure in otherexample embodiments. The anti-reflection layer 130 may be omitted, ifdesired.

The front electrode part 140 includes a plurality of front electrodes (aplurality of first electrodes) 141 and a plurality of front bus bars (aplurality of first bus bars) 142 connected to the plurality of frontelectrodes 141.

The plurality of front electrodes 141 are electrically and physicallyconnected to portions of the emitter part 121 and extend substantiallyparallel to one another in a predetermined direction at a distancetherebetween. The plurality of front electrodes 141 collect carriers(e.g., electrons) moving to the emitter part 121.

The plurality of front bus bars 142 are electrically and physicallyconnected to portions of the first emitter part 121 and extendsubstantially parallel to one another in a direction crossing anextending direction of the front electrodes 141.

The front electrodes 141 and the front bus bars 142 are placed on thesame level layer (or are coplanar). The front electrodes 141 and thefront bus bars 142 are electrically and physically connected to oneanother at crossings of the front electrodes 141 and the front bus bars142.

As shown in FIG. 1, the plurality of front electrodes 141 have a stripeshape extending in a transverse (or longitudinal) direction, and theplurality of front bus bars 142 have a stripe shape extending in alongitudinal (or transverse) direction. Thus, the front electrode part140 has a lattice shape on the front surface of the substrate 110.

By the front electrode part 140, the anti-reflection layer 130 is notpositioned on the portions of the emitter part 121 on which the frontelectrodes 141 and the front bus bars 142 are positioned, but ispositioned on portion of the emitter part 121 between portions the frontelectrode part 140 (for example, portions between the front electrodes141 and between the front bus bars 142).

The plurality of front bus bars 142 collect not only carrierstransferred from portions of the emitter part 121 contacting theplurality of front bus bars 142 but also the carriers collected by theplurality of front electrodes 141.

Because the plurality of front bus bars 142 collect the carrierscollected by the plurality of front electrodes 141 and move the carriersto a desired location, a width of each of the plurality of front busbars 142 is greater than a width of each of the plurality of frontelectrodes 141.

When the plurality of front bus bars 142 are connected to an externaldevice, the carriers (for example, electrons) collected by the front busbars 142 are output to the external device.

The front electrode part 140 including the plurality of front electrodes141 and the plurality of front bus bars 142 is formed of at least oneconductive material, for example, silver (Ag).

Although FIG. 1 shows a predetermined number of front electrodes 141 anda predetermined number of front bus bars 142 on the substrate 110, thenumber of front electrodes 141 and the number of front bus bars 142 mayvary.

The back surface field part 172 is a region (for example, a p*-typeregion) that is more heavily doped than the substrate 110 withimpurities of the same conductivity type as the substrate 110. Thus, theback surface field part 172 has a sheet resistance less than that of thesubstrate 110.

A potential barrier is formed by a difference between impurityconcentrations of a first conductivity region of the substrate 110 andthe back surface field part 172. Hence, the potential barrier preventsor reduces electrons from moving to the back surface field part 172 usedas a moving path of holes and makes it easier for holes to move to theback surface field part 172. Thus, an amount of carriers lost by arecombination and/or a disappearance of the electrons and the holes atand near the back surface of the substrate 110 is reduced, and amovement of carriers to the back electrode part 150 increases byaccelerating a movement of desired carriers (for example, holes).

The back electrode part 150 includes a back electrode (a secondelectrode) 151 and a plurality of back bus bars 152 connected to theback electrode 151.

The back electrode 151 contacts the back surface field part 172positioned at the back surface of the substrate 110 and is positioned onthe entire back surface of the substrate 110 except a formation area ofthe back bus bars 152. Thereby, the back electrode 151 is positioned ata portion of the back surface of the substrate 110, which is positionedbetween the back bus bars 152. However, if necessary or desired, theback electrode 151 may be not positioned at an edge of the back surfaceof the substrate 110 as well as the formation area of the back bus bars152.

The back electrode 151 contains a conductive material such as aluminum(Al).

The back electrode 151 collects carriers (for example, holes) moving tothe back surface field part 172.

Because the back electrode 151 contacts the back surface field part 172having the impurity concentration higher than the substrate 110, acontact resistance between the substrate 110 (i.e., the back surfacefield part 172) and the back electrode 151 decreases. Hence, the carriertransfer efficiency from the substrate 110 to the back electrode 151 isimproved.

The plurality of back bus bars 152 are positioned on the back surface ofthe substrate 110, on which the back electrode 151 is not positioned,and are connected to the back electrode 151.

Further, the plurality of back bus bars 152 are positioned opposite theplurality of front bus bars 142 with the substrate 110 therebetween.That is, the back bus bars 152 and the front bus bars 142 may bealigned, but such is not required.

The plurality of back bus bars 152 collect carriers from the backelectrode 151 in the same manner as the plurality of front bus bars 142.

The plurality of back bus bars 152 are connected to the external deviceand output the carriers (for example, holes) collected by the back busbars 152 to the external device.

The plurality of back bus bars 152 may be formed of a material havingbetter conductivity than the back electrode 151. Further, the pluralityof back bus bars 152 may contain at least one conductive material, forexample, silver (Ag).

Alternatively, the back electrode 151 may be positioned on the entireback surface of the substrate 110. In this instance, the back bus bars152 may be positioned opposite the front bus bars 142 with the substrate110 therebetween on the back electrode 151. Further, the back electrode151 may be positioned on substantially the entire back surface of thesubstrate 110 except the edge of the back surface of the substrate 110,if necessary or desired.

An operation of the solar cell 1 having the above-described structure isdescribed below.

When light irradiated to the solar cell 1 is incident on the emitterpart 121 and the substrate 110 through the anti-reflection layer 130, aplurality of electron-hole pairs are generated in the emitter part 121and the substrate 110 by light energy based on the incident light. Inthis instance, because a reflection loss of the light incident on thesubstrate 110 is reduced by the textured surface of the substrate 110and the anti-reflection layer 130, an amount of light incident on thesubstrate 110 further increases.

By the p-n junction of the substrate 110 and the emitter layer 121, theelectrons and the holes move to the n-type semiconductor part (forexample, the emitter part 121) and the p-type semiconductor part (forexample, the substrate 110), respectively. The electrons moving to then-type emitter part 121 are collected by the front electrodes 141 andthe front bus bars 142 and then move to the front bus bars 142. Theholes moving to the p-type substrate 110 are collected by the backelectrodes 151 and the back bus bars 152 and then move to the back busbars 152. When the front bus bars 142 are connected to the back bus bars152 using electric wires, current flows therein to thereby enable use ofthe current for electric power.

Referring to FIGS. 3A to 3G, a method for the solar cell 1 according toan example embodiment of the invention is described.

As shown in FIG. 3A, by using a dry etching method such as a reactionion etching (RIE) method, etc., or a wet etching method, an exposedsurface, for example, a front surface of a substrate 110 of a firstconductivity type is etched to form a textured surface (an unevensurface) having a plurality of projections 11 and a plurality ofdepressions 12. A projected height H1 of each projection 11 and a widthH2 of each projection 11 are of various magnitudes, respectively.

Next, as shown in FIG. 3B, by an ion implantation method, impurities areinjected into only the front surface of the substrate 110 having theuneven surface, to form an impurity region 120. Since the impurities areof a second conductivity, for example, a n-type, opposite the firstconductivity type, the impurity region 120 is of the second conductivitytype. The impurity region 120 is a region obtained by injecting ions(that is, impurity ions) of the n-type impurities into the substrate 110using the ion implantation method, and thereby, the impurity region 120has a thickness measured from the front surface of the substrate 110. Anenergy for injecting the impurity ions may be about 5KeW to 30KeW.

When the impurity ions are injected into the substrate 110, the impurityions come into collision with the front surface of the substrate 110 andthereby a damage portion 21 in which normal silicon bonds in thesubstrate 110 are broken is formed at and/or near the front surface ofthe substrate 110, that is, a surface of the impurity region 120. Thedamage portion 21 may be totally or partially formed at and/or near theentire front surface of the substrate 110 exposed to the impurity ionsfor the ion implantation method. The damage portion 21 is mainly formedat and near the surface of the substrate 110 but may be formed into theimpurity region 120.

Since the injection of the impurity ions is performed only at the frontsurface of the substrate 110 which is exposed to the impurity ions,unlike a thermal diffusion method, the impurity region 120 is not formedat a back surface of the substrate 110.

As shown in FIG. 3C, the substrate 110 is heated in an atmosphere of anitrogen gas (N₂) to activate the impurity region 120 formed in thesubstrate 110, and to thereby, an emitter region 1201. That is, by theactivation process using the heat treatment, the impurity ions existingin the impurity region 120 in an interstitial state are reconfiguredwith silicon (Si) such that a state of the impurity ions is changed fromthe interstitial state into a substitutional state. Thereby, theimpurity region 120 functions as an emitter part of a p-type or ann-type and thereby, forms form a p-n junction with the firstconductivity portion of the substrate 110.

Further, by the heat treatment, the impurity ions existing in theimpurity region 120 is more deeply diffused into the substrate 110, andthereby a thickness (a depth) (that is, an impurity doped thickness) ofthe emitter region 1201 that is the activated impurity region is greaterthan a thickness of the impurity region 120. The heat treatment may beperformed at about 800° C. to 1100° C. For example, when the impurityregion 120 is of an n-type, for example, containing phosphorous (P), theheat treatment may be performed at about 800° C. to 1100° C. and whenthe impurity region 120 is of a p-type, for example, containing boron(B), the heat treatment may be performed at about 900° C. to 1100° C.

An example of a variation of an impurity doped concentration of theemitter region 1201 is shown in a graph G1 of FIG. 4. The graph G1 ofthe FIG. 4 shows the impurity doped concentration in accordance with athickness change of the emitter region 1201 from a surface of theemitter region 1201 to the back surface of the substrate 110.

As shown in the graph G1, the impurity doped concentration from thesurface of the emitter region 1201 to a thickness of about 0.07 μmadjacent to the surface of the emitter region 1201 increases from about4.0E+19 cm⁻³ to about 6.5E+19 cm⁻³ and the impurity doped concentrationof the emitter region 1201 from a thickness of about 0.07 μm to athickness about 0.48 μm gradually decreases from about 6.5E+19 cm⁻³ toabout 4.0E+19 cm⁻³.

During the heat treatment for activating the impurity region 120, arecrystallization of silicon Si is performed at a recrystallizationtemperature of silicon, whereby damaged silicon lattices of the damagedportion 21 may be reconfigured. When the silicon recrystallization hasoccurred, the damaged silicon lattices in the damage portion 21 may bereconfigured into stable silicon lattices to anneal the damaged siliconlattices.

The activation process of the impurity region 120 may be performed in anatmosphere of an oxygen (O₂) gas. In this instance, a silicon oxidelayer may be formed on the impurity region 120 by coupling oxygen of theatmosphere and silicon of the impurity region 120. The silicon oxidelayer may have a thickness of about 15 nm, to 30 nm.

When forming the emitter region by using the ion implantation method,the damage portion existing on the emitter region is shown in FIG. 5.

FIG. 5 depicts a portion of a solar cell formed with an anti-reflectionlayer after forming an emitter region by the ion implantation andactivation processes by using a TEM (transmission electron microscopy)equipment. The emitter region having the damage portion functions as anemitter part.

In FIG. 5, a reference numeral {circle around (4)} denotes a siliconsubstrate, a reference numeral {circle around (3)} denotes the damageportion formed at a surface of the silicon substrate, a retferencenumeral {circle around (2)} denotes a native silicon oxide portiongenerated by the exposure to an air, and a reference numeral {circlearound (1)} denotes the anti-reflection layer.

Next, referring to FIG. 3D, a surface (a front surface which is asurface opposite a p-n junction surface with the substrate 110 and theemitter region 1201) of the emitter region 1201, is removed to form anemitter part 121. By the removal, the entire surface of the emitterregion 1201 is removed by a predetermined thickness and thereby thedamage portion 21 formed at and/or near the surface of the emitterregion 1201 is also removed. An impurity doped concentration of theemitter part 121 may be about 1×10¹⁹ cm⁻³ to 1×10²⁰ cm⁻³.

A removed thickness of the impurity region 120 may be about 5 nm to 35nm. For example, when the activation process is performed in thenitrogen atmosphere, a separate layer such as the silicon oxide layer isnot formed on the emitter region 1201 activated by the nitrogen N₂.Thus, the emitter region 1201 is removed by a thickness of about 5 nm to20 nm to further remove an amount greater than a thickness of the damageportion 21 existing at and/or near the surface of the emitter region1201. However, when the activation process is performed in the oxygenatmosphere, since the silicon oxide layer formed on the emitter region1201 as well as the damage portion 21 should be removed, the removalthickness increases. Thus, for example, the removed thickness of theemitter region 1201 may be about 20 nm to 35 nm.

As described above, since the emitter region 1201 is etched by anetchant and removed, and the entire front surface of the emitter region1201 is exposed to the etchant, the thickness of the emitter portion 121is reduced by the thickness removed from the emitter region 1201. Inthis instance, the removed thickness of the emitter region 1201 may beadjusted using a concentration of the etchant and/or an etching time,etc.

By the removal of a portion of the emitter region 1201 adjacent to thesurface thereof, into which the impurity ions are injected, the impuritydoped concentration of the emitter part 121 in accordance with a depthchange of emitter portion 121 is varied as a graph G2 of FIG. 4.

For example, as shown in the graph G2 of FIG. 4, when a thickness ofemitter part 121 from the surface of emitter part 121 is about 0.01 μm,the impurity doped concentration of the emitter part 121 has the maximumvalue, about 6.0E+19 cm⁻³, and then gradually decreases from about6.0E+19 cm⁻³ to about 1.0E+19 cm⁻³. Thereby, the minimum impurity dopedconcentration of the emitter part 121 is about 1.0E+19 cm⁻³ and thethickness of the emitter part 121 having the minimum impurity dopedconcentration is about 0.42 μm.

Since the front surface of the emitter region 1201, at and/or near whichthe damage portion 21 exists is removed by a predetermined thickness,the thickness of the emitter part 121 is less than that of the emitterregion 1201.

Thus, referring to FIG. 4, when the front surface of the emitter part1201 is not removed, the thickness of the emitter region 1201 is about0.47 μm, and the thickness of the emitter part 121 is about 0.42 μmafter the front surface of the emitter region 1201 is removed.

For removing the entire front surface of the emitter region 1201including the damage portion 21 by the predetermined thickness, an etchprevention layer is formed on a desired portion (for example, a backsurface of the substrate 110 on which the etching is not desired) of thesubstrate 110 and then the substrate 110 is immersed into the etchant.Thus, the entire front surface of the emitter region 1201 on which theetch prevention layer is not formed is removed by the predeterminedthickness, to remove the damage portion 21 existing at and/or near thesurface of the emitter region 1201.

Alternatively, without the formation process of the etch preventionlayer, the emitter region 1201 of the predetermined thickness may beremoved by immersing a desired thickness of only the front surface ofthe substrate 110 (that is, the front surface of the emitter region1201) into the etchant.

The etchant for removing the emitter region 1201 may be composed ofnitric acid HNO₃, hydrofluoric acid HF and pure wafer. The nitric acidHNO₃ is used to oxygenate silicon composing the emitter region 1201 andthe hydrofluoric acid HF is used to remove the oxygenated silicon.

When the heat treatment for activating the impurity region 120 isperformed in the oxygen (O₂) atmosphere, the nitric acid HNO₃ may beomitted from the etchant. That is, since the impurity region 120 isoxygenated by oxygen (O₂) supplied during the heat treatment, the nitricacid HNO₃ oxygenating silicon Si is possible to omit. Further, asalready described, the silicon oxide layer generated by the oxygenationof silicon on the impurity region 120 should be removed along with theemitter region 120, at least one of a concentration of the hydrofluoricacid HF and an etching time may be increased as compared with a case inwhich the impurity region 120 is activated in the nitrogen (N₂)atmosphere.

By the etching process of the emitter region 1201, the impurity dopedconcentration of the emitter portion 121 has the maximum value at and/ornear the surface of the emitter portion 121 contacting the frontelectrode part 140. Thus, conductivity of regions of the emitter portion121 contacting the front electrode part 140 increase. In addition, sincethe thickness of the emitter portion 121 decreases, charge transferdistances of charges moving to the surface of the emitter portion 121are reduced. Thus, the charges further easily move from the emitterportion 121 to the front electrode part 140 adjacent to the emitterportion 121.

After forming the emitter portion 121 using the ion implantation methodincluding the ion implantation process, the activation process and theetching process, as shown in FIG. 3E, an anti-reflection layer 130 isformed on the emitter part 121 on the front surface of the substrate 110using a plasma enhanced chemical vapor deposition (PECVD), etc. In thisexample, the anti-reflection layer 130 may be made of hydrogenatedsilicon nitride (SiNx) or hydrogenated silicon oxide (SiOx) etc.

Next, referring to FIG. 3F, a paste containing metals such as silver(Ag) is printed on corresponding portions of the anti-reflection layer130 using a screen printing method and then is dried to form a frontelectrode part pattern 40.

The front electrode pattern 40 includes a front electrode pattern 41 anda front bus bar pattern 42.

Next, referring to FIG. 3G, a paste containing metals such as aluminwn(Al) is selectively or partially printed on the back surface of thesubstrate 110 using a screen printing method and then is dried to form aback electrode pattern 51 and a paste containing metals such as silver(Ag) is printed on portions of the back surface of the substrate 110 onwhich the back electrode pattern 51 is not formed using a screenprinting method and then is dried to form a back bus bar pattern 52.Thereby, a back electrode part pattern 50 having the back electrodepattern 51 and the back bus bar pattern 52 is completed. The back busbar pattern 52 is opposite the front bus bar pattern 42 with thesubstrate 110 therebetween.

The patterns 40 and 50 may be dried at about 120° C. to 200° C., and aformation order of the patterns 40 and 50 may be changed.

Next, the substrate 110 having the patterns 40 and 50 is heated at about750° C. to 800° C.

By the heat treatment, a front electrode part 140 having a plurality offront electrodes 141 and a plurality of front bus bars 142 electricallyand physically connected to the emitter part 121, a back surface fieldpart 172 at the back surface of the substrate 110 on which the backelectrode pattern 51 is formed, and a back electrode part 150 includinga back electrode 151 electrically connected to the substrate 110 throughthe back surface field part 172 and a plurality of back bus bars 152electrically and physically connected to the substrate 110 and the backelectrode 151 are formed, to complete a solar cell 1 (refer to FIGS. 1and 2).

By the heat process, by an etching material such as lead (or PbO)contained in the front electrode pattern 41, the front electrode pattern41 penetrates through portions of the anti-reflection layer 130underlying the front electrode pattern 41 and is connected to theemitter part 121, thereby forming the plurality of front electrodes 141and the plurality of front bus bars 142, to complete the front electrodepart 140.

In this instance, the front electrode pattern 41 of the front electrodepart pattern 40 is formed as the plurality of front electrodes 141 ofthe front electrode part 140, and the front bus bar pattern 42 of thefront electrode part pattern 40 is formed as the plurality of front busbars 142 of the front electrode part 140.

In addition, during the heat process, the back electrode pattern 51 andthe back bus bar pattern 52 of the back electrode part pattern 50 areformed as the back electrode 151 and the plurality of back bus bars 152,respectively, and aluminum (Al) contained in the back electrode pattern51 of the back electrode part pattern 50 is diffused (or doped) into thesubstrate 110 to form an impurity region, that is, the back surfacefield part 172 that is highly doped with impurities of the sameconductivity type as the substrate 110. In this instance, an impuritydoped concentration of the back surface field region 172 is higher thanthat of the substrate 110. The back electrode pattern 51 and the backbus bar pattern 52 do not contain an etching material (e.g., Pb). Eventhough the back electrode pattern 51 and the back bus bar pattern 52contain the etching material, the back electrode pattern 51 and the backbus bar pattern 52 contains the etching material equal to or less than apredetermined amount (e.g., 1000 ppm) not influencing the etching of theunderlying layer (that is, the substrate of the back electrode pattern51 and the back bus bar pattern 52. Thus, unlike the front electrodepart pattern 40, portions of the substrate 110 contacting the backelectrode pattern 51 and the back bus bar pattern 52 are not etchedduring the thermal treatment process. Thereby, the back electrode 151 isin contact with the back surface field part 172 to be electricallyconnected to the substrate 110.

In this example, since by the ion implantation method, the emitter part121 is formed only at (in) the front surface of the substrate 110 and isnot formed at (in) the back surface of the substrate 110,characteristics of the back surface field part 172 are improved.

That is, when the emitter part 121 having the opposite conductivity typeto the conductivity type of the substrate 110 is positioned at the backsurface of the substrate 110, impurities having the oppositeconductivity type and contained in the emitter part 121 are mixed to theback surface field part 172 of the same conductivity type as thesubstrate 110, and thereby a field effect by the back surface field part172 is weakened.

However, in this instance, since the emitter part 121 is not formed atthe back surface of the substrate, the reduction of the field effect ofthe back surface field part 172 due to the emitter part 121 does notoccur and the field effect of the back surface field part 172 is furtherimproved. Thus, an amount of charges moving to the back surface of thesubstrate 110 increases to improve an efficiency of the solar cell 1.

Moreover, in performing the heat process, metal components contained inthe patterns 40 and 50 are chemically coupled to the contacted emitterpart 121 and the substrate 110, respectively, such that a contactresistance is reduced and thereby a charge transfer efficiency isimproved to improve a current flow.

Further, since the emitter part 121 is formed at only the front surfaceof the substrate 110, an edge isolation process separating the emitterpart 121 formed in the front surface of the substrate 110 and theemitter part formed in the back surface of the substrate 110 or aseparate process for removing the emitter part formed in the backsurface of the substrate 110 are not necessary. Thus, a manufacturingtime of the solar cell 1 is reduced to increase productivity of thesolar cell 1 and a manufacturing cost of the solar cell 1 is alsoreduced.

Next, referring to FIGS. 6 and 7, a solar cell according to anotherexample embodiment of the invention is described.

A solar cell 2 shown in FIGS. 6 and 7 has the same structure as thesolar cell of FIGS. 1 and 2 except an emitter part 121 a and a frontelectrode part 140 connected to the emitter part 121 a.

That is, the emitter part 121 a includes first and second emitterportions 1211 and 1212 each having different impurity dopedconcentrations and different impurity doped thicknesses from each other.Thus, the solar cell 2 of the example includes a selective emitterstructure.

In this example, the impurity doped thicknesses of the first emitterportion 1211 is less than the impurity doped thicknesses of the secondemitter portion 1212, and thereby, the impurity doped concentration ofthe first emitter portion 1211 is less than the impurity dopedconcentration of the second emitter portion 1212. Thus, conductivity ofthe second emitter portion 1212 is greater than that of the firstemitter portion 1211 and a sheet resistance of the second emitterportion 1212 is less than that of the first emitter portion 1211.

An amount (that is, a damage amount) of damage portion generated atand/or near a surface of the second emitter portion 1212 in injectingimpurity ions is more than an amount (a damage amount) of damage portiongenerated at and/or near a surface of the first emitter portion 1211.Thus, the damage amount existing at the second emitter portion 1212 ismore than that of the first emitter portion 1211. In particular, thedamage amount existing at and/or near the surface of the second emitterportion 1212 is more than that at and near the surface of the firstemitter portion 1211.

When comparing a TEM photograph of the second emitter portion 1212 and aTEM photograph of the first emitter portion 1211, the damage amountobserved in the TEM photograph of the second emitter portion 1212 ismore than that observed in the TEM photograph of the first emitterportion 1211. Thus, the damage amounts of the first and second emitterportions 1211 and 1212 may be measured by using a TEM equipment. Asanother method for measuring the damage amount of the first and secondemitter portions 1211 and 1212, a CV (capacitance voltage) measurementequipment may be used. For example, charge mobility or leakage currentin the first and second emitter portions 1211 and 1212 is measured bythe CV measurement equipment and the damage amounts of the first andsecond emitter portions 1211 are calculated based on the charge mobilityor leakage current. In general, as the damage amount increases, theleakage current increases and the charge mobility decreases.

In this example, the front electrode part 140 is connected to the secondemitter portion 1212 of the emitter part 121 a having the conductivitymore than that of the first emitter portion 1211.

Since the front electrode part 140 is connected to the second emitterportion 1212, charges transferring to the emitter part 121 a move to thesurface of the first emitter portion 1211 and then move to the frontelectrode part 140 along the surface of first emitter portion 1211. Inthis instance, since the impurity doped thickness of the first emitterportion 1211 is less than that of the second emitter portion 1212, thecharge transfer distances of charges moving to the surface of the firstemitter portion 1211 are reduced. Thus, an amount of charges collectedby the front electrode part 140 increases and an efficiency of the solarcell 2 is improved.

Since the first emitter portion 1211 through which charges mainly moveto adjacent portions of the front electrode part 140 has the impuritydoped concentration less than that of the second emitter portion 1212,when the charges move from the first emitter portion 1211 to the secondemitter portion 1212, a loss amount of the charges due to the impuritiesof the first emitter portion 1211 decreases and mobility of the chargesincreases. Thus, an amount of charges moving from the first emitterportion 1211 to the second emitter portion 1212 increases.

Further, since the front electrode part 140 is connected to the secondemitter portion 1212 having larger conductivity and less resistance thanthe first emitter portion 1211, a charge transfer efficiency from thesecond emitter portion 1212 and the front electrode part 140 isincreased. Thus, the efficiency of the solar cell 1 is improved.

As described above, the impurity doped concentration of the secondemitter portion 1212 formed by the ion implantation method is greaterthan that of a second emitter portion formed by the thermal diffusionmethod. Thus, the conductivity of the second emitter portion 1212contacting the front electrode part 140 is greater than that of thesecond emitter portion formed by the thermal diffusion method, andthereby, contact resistance between the second emitter portion 1212 andthe front electrode part 140 further decreases. Thereby, an amount ofcharges transferring from the second emitter portion 1212 to the frontelectrode part 140 increases and an amount of charges collected by thefront bus bars 142 also increases.

As described above, since the first and second emitter portions 1211 and1212 are different impurity doped thicknesses from each other, adistance (hereinafter, referred to as ‘a first shortest distance’) d1from the front surface of the substrate 110 to a p-n junction surface(hereinafter, referred to as ‘a first junction surface’) between thefirst emitter portion 1211 and the substrate 110 is different from adistance (hereinafter, referred to as ‘a second shortest distance’) d2from the front surface of the substrate 110 to a p-n junction surface(hereinafter, referred to as ‘a second junction surface’) d2 between thesecond emitter portion 1212 and the substrate 110. That is, as shown inFIGS. 6 and 7, the first shortest distance d1 is shorter than the secondshortest distance d2.

In the substrate 110, the first and second junction surfaces arepositioned at the same level (i.e., the same height) as each other.Thus, a third shortest distance from the back surface of the substrate110 to the first junction surface is substantially equal to a fourthshortest distance from the back surface of the substrate 110 to thesecond junction surface. In this instance, the first to fourth shortestdistances are substantially equal to each other within the margin oferror obtained by a difference between the heights of each projection ofthe textured surface of the substrate 110.

Since the emitter part 121 is formed at the front surface of thesubstrate which is the textured surface, the junction surface betweenthe substrate 110 and the emitter part 121 a may be not a flat surfacebut an uneven surface under the influence of the textured surface of thesubstrate 110.

As described above, since the front electrode part 140 is connected toonly the second emitter portion 1212 of the emitter part 121 a, theanti-reflection layer 130 is mainly positioned on the first emitterportion 1211 of the emitter part 121 a positioned between portions ofthe front electrode part 140.

The emitter part 121 a may be formed as discussed below.

As described referring to FIGS. 3A to 3C, after forming a texturedsurface at a front surface of the substrate 110, an emitter region 1201is formed by an ion implantation method and an activation process. Theemitter region 1201 has a damage portion 21 existing at at least one ofa surface of the emitter region 1201.

Next, as shown in FIG. 8A, an etch protection layer 81 is selectively orpartially formed on the emitter region 1201 to expose portions of theemitter region 1201 on which the etch protection layer 81 is not formed.

As shown in FIG. 8B, the front surface of the substrate 110 is exposedto an etchant, to remove the exposed portions of the emitter region 1201by a predetermined thickness (a desired thickness).

The etchant may be composed of nitric acid HNO₃, hydrofluoric acid HFand pure wafer, etc., or may be composed of the hydrofluoric acid HF andpure wafer, excluding the nitric acid HNO₃.

Thus, a portion etched of the emitter region 1201 is formed as the firstemitter portion 1211 and a portion not etched of the emitter region 1201is formed as the second emitter portion 1212, to form an emitter part121 a (FIG. 8B). Then, the etch protection layer 81 is removed by acleansing liquid such as water, etc. Since a reason for removing theportions of the emitter region 1201 is to form the first emitter portion1211 having a thickness less than that of the second emitter portion1212, a removed thickness of the emitter region 1201 may be about 30 nmto 100 nm.

The second emitter portion 1212 includes a portion 1212 a (a frontelectrode second emitter portion) for a plurality of front electrodesextending in a predetermined direction and a portion (a front bus barsecond emitter portion) 1212 b for a plurality of front bus barsextending in a direction crossing the front electrode second emitterportion 1212 a. A width of the front bus bar second emitter portion 1212b is greater than that of the front electrode second emitter portion1212 a.

In an alternative example, the etch protection layer 81 may be formed onthe back surface as well as the front surface of the substrate 110, andthen the entire surface of the substrate 110 may be exposed to theetchant to form a selective emitter structure having the first andsecond emitter portions 1211 and 1212.

Thereby, since the damage portion 21 existing at and/or near the frontsurface of the emitter region 1201 for the first emitter portion 1211 isremoved, the first emitter portion 1211 may have an impurity dopinggraph such as a shape of the graph G2 of FIG. 4, an impurity dopedthickness of first emitter portion 1211 is reduced by the removedthickness of the emitter region 1201. Thereby, an impurity dopedthickness of the first emitter portion 1211 is less than that of thesecond emitter portion 1212 at which the etching is not performed. Inthis instance, the second emitter portion 1212 may have an impuritydoping graph such as a shape of the graph G1 of FIG. 4 since portions ofthe emitter region 1201 corresponding to the second emitter portion 1212are not removed. Thereby, since the surface of the emitter region 1201having a higher impurity doped concentration than other portion of theemitter region 1201 is removed, a surface impurity doped concentrationat the surface of the first emitter portion 1211 is less than that ofthe second emitter portion 1212. For example, the surface impurity dopedconcentration of the first emitter portion 1211 may be about 2×10¹⁹ cm⁻³and the surface impurity doped concentration of the second emitterportion 1212 may be about 4×10¹⁹ cm⁻³.

Thus, the impurity doped thickness of the first emitter portion 1211 isreduced, and the charge transfer distance of charges moving to thesurface of the first emitter portion 1211 decreases.

Next, as shown in FIG. 8C, an anti-reflection layer 130 is formed on theentire front surface of the substrate 110, that is, on the first emitterportion 1211 and the second emitter portion 1212 of the emitter part 121a.

Since the anti-reflection layer 130 is positioned on the first emitterportion 1211 at which the damage portion 21 of the emitter region 1201is removed, both a reduction effect of a charge loss due to the damageportion 21 and a passivation effect by the anti-reflection layer 130 areobtained at the first emitter portion 1211. Thus, a loss amount ofcharges moving from first emitter portion 1211 to the second emitterportion 1212 is further reduced.

As described referring to FIG. 8D, a paste containing silver (Ag) and anetching material is printed on the second emitter portion 1212 and thenis dried to form a front electrode part pattern 40 having a frontelectrode pattern 41 and a front bus bar pattern 42, and a pastecontaining aluminum (Al) and a paste containing silver (Ag) are printedon the back surface of the substrate 110 and then are dried to form aback electrode part pattern 50 having a back electrode pattern 51 and aback bus bar pattern 52 (referring to FIG. 8D.) The front electrodepattern 41 is formed on and along the front electrode second emitterportion 1212 a and the front bus bar pattern 42 is formed on and alongthe front bus bar second emitter portion 1212 b.

Then, in the same manner as described above, by heating the substrate110 with the pattern 40 and 50, a front electrode part 140 having aplurality of front electrodes 141 and a plurality of first bus bars 142penetrating the anti-reflection layer 130 and contacting the secondemitter portion 1212, a back surface field part 172, that is highlydoped with impurities of the same conductivity type as the substrate110, a back electrode part 150 including a back electrode 151 contactingthe back surface field part 172 and electrically connected to thesubstrate 110 through the back surface field part 172 and a plurality ofback bus bars 152 electrically and physically connected to the substrate110 and the back electrode 151 are formed. Thereby, the solar cell 2 iscompleted.

When an activation process for forming the emitter region 1201 isperformed in an oxygen atmosphere, a silicon oxide layer may be formedon the second emitter portion 1212. However, when the front electrodepattern 40 penetrates the anti-reflection layer 130 for forming thefront electrode part 140, the silicon oxide layer underlying theanti-reflection layer 130 is also penetrated by the front electrode partpattern 40. Thereby, the silicon oxide layer generated during theactivation process does not negatively influence the contact between thefront electrode part 140 and the emitter part 121 a.

Another example of the selective emitter structure of the solar cell isshown in FIG. 9.

As shown in FIG. 9, an emitter part 121 a having first and secondemitter portions 1211 and 1212 has a different structure from theemitter part 121 a of FIGS. 6 and 7.

That is, in FIGS. 6 and 7, a p-n junction surface of the first emitterportion 1211 is the same level as a p-n junction surface of the secondemitter portion 1212, and the second emitter portion 1212 having theimpunity doped thickness greater than that of the first emitter portion1211 is projected toward the substrate 110. Thus, a surface (i.e., afront surface, that is, an opposite surface of the p-n junction surface)of the first emitter portion 1211 and a surface (i.e., a front surface,that is, an opposite surface of the p-n junction surface) of the secondemitter portion 1212 are not positioned at the same level (i.e., thesame height) as each other and thereby, the surface (the front surface)of the emitter part 121 a is not a flat surface but an uneven surfacehaving projections at which the second emitter portion 1212 is projectedto the incident surface of the substrate 110. Thereby, a front surfaceposition of the second emitter portion 1212 is higher than that of thefirst emitter portion 1211.

However, in FIG. 9, a surface (i.e., a front surface) of the firstemitter portion 1211 and a surface (i.e., a front surface) of the secondemitter portion 1212 are positioned at the same level (i.e., the sameheight) and thereby, the front surface of the emitter part 121 a issubstantially level. However, a p-n junction of the first emitterportion 1211 and a p-n junction of the second emitter portion 1212 arenot of the same level, but are positioned at different positions (i.e.,different heights), respectively. Thus, a front position of the firstemitter portion 1211 is equal to the front position of the secondemitter portion 1212 and the p-n junction surface of the second emitterportion 1212 is projected from the p-n junction of the first emitterportion 1211 to the back surface of the substrate 110. The p-n junctionof the emitter part 121 a is an uneven surface having projections atwhich the second emitter portion 1212 is projected to the back surfaceof the substrate 110.

Functions of the first and second emitter portions 1211 and 1212 of thesolar cell shown in FIG. 9 are equal to that of the first and secondemitter portions 1211 and 1212, and thereby the functions of the firstand second emitter portion 1211 and 1212 are not described in detail.

A method for manufacturing the emitter part 121 a is described below.

As described referring to FIG. 10A, first and second impurity portions120 a and 120 b which are of a second conductivity type are formed at afront surface of the substrate 110 by injecting impurity ions of thesecond conductivity type using an ion implantation method. The firstimpurity portion 120 a has different impurity doped concentration andimpurity doped thickness from the second impurity portion 120 b. Thefirst and second impurity portions 120 a and 120 b are an impurityregion.

The first and second impurity portions 120 a and 120 b having differentimpurity doped thicknesses from each other are formed by using a mask85. The mask 85 may be equipped to an ion implantation equipment for theion implantation method. An amount of ions injected into portions of thesubstrate 110 over which the mask 85 is positioned is less than anamount of ion injected into the remaining portion of the substrate 110over which the mask 85 is not positioned. Thus, the portions of thesubstrate 110 corresponding the mask 85 is formed as the first impurityportion 120 a, and the remaining portion of the substrate 110 (that is,portions over which the mask 85 is not positioned) are formed as thesecond impurity portion 120 b.

As another example for manufacturing the first and second impurityportions 120 a and 120 b, the mask 85 may be formed on the entire frontsurface of the substrate 110. In this instance, an exposed area of thesubstrate 110 exposed through the mask 85 in a unit area thereof ischanged depending on positions of the substrate 110. For example, themask 85 may include a first portion having a first exposed area of thesubstrate 110 in the unit area and a second portion having a secondexposed area of the substrate 110 in the unit area, and the secondexposed area is greater than the first exposed area. Thereby, when, byusing an ion implantation method, impurity ions are applied on the mask85 having the first and second portions and positioned over the entirefront surface of the substrate 110, a portion of the substrate 110facing the first portion of the mask 85 may be formed as the firstimpurity portion 120 a having a first impurity doped thickness(concentration) and a portion of the substrate 110 facing the secondportion of the mask 85 may be formed as the second impurity portion 120b having a second impurity doped thickness (concentration) greater thanthe first impurity doped thickness (concentration).

The mask 85 having the first and second portions may be positioneddirectly on the substrate 110, and the first and second impurityportions 120 a and 120 b may be formed at the substrate 110 in the samemanner as that described above.

Alternatively, after forming a first impurity portion 120 a having adesired impurity doped thickness at the entire front surface of thesubstrate 110, impurity ions being further selectively or partiallyinjected at the first impurity portion 120 a by using a mask such thatportions of the first impurity portion 120 a are formed as the secondimpurity portion 120 b. In this instance, the second impurity portion120 b is the portion of the substrate 110 at which the impurity ions arefurther injected, and the first impurity portion 120 a is the remainingportion of the substrate 110 at which the impurity ions are not furtherinjected.

Further, the first and second impurity portions 120 a and 120 b may beformed by using various known methods using the ion implantation method.

Since the first and second impurity portions 120 a and 120 b are formedby using the ion implantation method, a damage portion 21 due to theimpurity ions is generated at and/or near surfaces (front surfaces) ofthe first and second impurity portions 120 a and 120 b.

Next, as shown in FIG. 10B, the front surface of the substrate 110having the first and second impurity portions 120 a and 120 b is heatedin a nitrogen (N₂) or oxygen (O₂) atmosphere and thereby the first andsecond impurity portions 120 a and 120 b are formed as first and secondemitter regions 12011 and 12012 of the emitter region 1201 a,respectively. In this instance, the damage portion 21 still exists atand/or near a front surface of the emitter region 1201 a.

Next, the entire front surface of the emitter region 1201 a is etched byan etchant and then removed to a desired thickness (a predeterminedthickness). In this instance, the damage portion 21 existing at and/ornear the front surface of the emitter region 1201 a is removed by theetchant. Thereby, an emitter part 121 a having the first and secondemitter portions 1211 and 1212 is completed (FIG. 10C). Since theremoval of the emitter region 1201 a is enough to remove the portion atand/or near the entire front surface of the emitter region 1201 a, atwhich the damage portion 21 largely exists is removed, a removedthickness of the emitter region 1201 a may be 5 nm to 35 nm.

The thicknesses of the first and second emitter portions 1211 and 1212of the emitter part 121 a are less than thicknesses of the first andsecond emitter regions 12011 and 12012 of the emitter region 1201 a,respectively, and the damage portion 21 is removed and does not exist atand/or near not only the first emitter portion 12011 but also at thesecond emitter portion 12012.

Since all the first and second emitter regions 12011 and 12012 areremoved by the etchant, variations of the impurity doped concentrationsof the first and second emitter portions 1211 and 1212 in accordancethickness variations of the first and second emitter portions 1211 and1212 have shapes as the graph G2 of FIG. 4, respectively. Further, asurface impurity doped concentration of the first emitter portion 1211is less than a surface impurity doped concentration of the secondemitter portion 1212.

Next, formation processes of an anti-reflection layer 130, a frontelectrode part 140, a back electrode part 150, and a back surface fieldpart 172 are equal to the processes described referring to FIGS. 8C and8D.

The process for forming the emitter part 121 by using the ionimplantation process, the activation process and the etching process maybe adopted for manufacturing the back surface field part 172.

That is, after forming an impurity portion of a first conductivity typeat a back surface of the substrate 110 by injecting impurity ions of thefirst conductivity type into the back surface of the substrate 110 inthe same manner as described above, the impurity portion of the firstconductivity type is activated by heating at a predetermined temperature(for example, about 800° C. to 1100° C.) to form the impurity portion ofthe first conductivity type into a back surface field region. The backsurface field region includes a damage portion due to the impurity ionsat and/or near a surface of the back surface field region. Then, theback surface field region is removed by a predetermined (desired)thickness from the surface of back surface field region to form the backsurface field part 172 at the back surface of the substrate 110. Theremoval of the back surface field region is performed by the etchantdescribed above, and the damage portion of the back surface field regionis removed during removing of the back surface field region.

For reducing or preventing deterioration of the substrate 110 by theheat treatment process for the activation process, after forming theimpurity portion of the second conductivity type for the emitter part121 and the impurity portion of the first conductivity type for the backsurface field part 172 at the front and back surfaces of the substrate110, respectively, the impurity portions formed at the front and backsurfaces of the substrate 110 may be simultaneously activated by oneheat treatment process to form the emitter part 121 and the back surfacefield part 172.

Since the back surface field part 172 is not formed during forming ofthe back electrode part 150 but formed by a separate process such as theion implantation process, the back surface field part 172 is furtherstably formed and the impurity doped concentration of the back surfacefield part 172 is further accurately controlled. As described above,when the back surface field part 172 is formed by the ion implantationmethod, a surface impurity doped concentration of the back surface fieldpart 172 is greater than that of a back surface field part formed byheat applied during forming the back electrode part 150, and thereby,contact resistance between the back surface field part 172 and a backelectrode 151 is reduced to increase an amount of charges moving fromthe substrate 110 to the back electrode 151.

After at least one of the emitter part 121 and the back surface fieldpart 172 is formed by the ion implantation process, the activationprocess and the etching process, an anti-reflection layer 130, a frontelectrode part 140 and a back electrode part 150 are formed as describedreferring to FIGS. 3E to 3G. Since the back surface field part 172 isalready formed before the formation of the front and back electrodeparts 140 and 150, the further generation of the back surface field part172 should be prevented during forming the back electrode 151. Forexample, a material of a back electrode pattern for forming the backelectrode 151 may be changed, or after the formation of the frontelectrode part 140, the back electrode part 150 should be formed at alow temperature not to further generate the back surface field part 172by the back electrode pattern.

Further, as shown in FIG. 11, at least one of the emitter part 121 a andthe back surface field part 172 a formed by the ion implantationprocess, the activation process, and the etching process is applied to abifacial solar cell of which light is incident on front and backsurfaces.

As shown in FIG. 11, in the same method as shown in FIGS. 6 and 7 andreferring to FIGS. 8A to 8C and 10A to 10C, the emitter pan 121 a of theselective emitter structure includes the first and second emitterportions 1211 and 1212, and a back surface field part 172 a of aselective back surface field structure includes first and second backsurface field portions 1721 and 1722.

All the first and second back surface field portions 1721 and 1722 haveimpurity doped concentrations greater than an impurity dopedconcentration of the substrate 110. The second back surface fieldportion 1722 has the impurity doped concentration greater than that ofthe first back surface field portions 1721, and thus the second backsurface field portion 1722 has conductivity greater than that of thefirst back surface field portions 1721.

Like a plurality of front electrodes 141, the back electrode includes aplurality of electrodes 151 extending substantially parallel to oneanother in a predetermined direction (that is, in the same direction asthe front electrodes 141) at a distance therebetween, and a back bus baralso includes a plurality of back bus bars 152 extending in a direction(that is, in the same direction as the front bus bars 142) crossing theplurality of back electrodes 151. In this instance, the plurality ofback electrodes 151 may face the plurality of front electrodes 141 withthe substrate 110 therebetwen and the plurality of back bus bars 152 mayface the plurality of front bus bars 142 with the substrate 110therebetwen.

When the solar cell have the selective back surface field structure,contact resistance between the second back surface field portion 1722and the back electrodes 151 is reduced and an amount of charges movingfrom the second back surface field portion 1722 to the back electrodes151 increases, and when the charges move along a surface of the firstback surface field portion 1721 to the second back surface field portion1722, a loss amount of the charges due to the impurities is decreased,to increase an amount of the charges moving from the first back surfacefield portion 1721 to the second back surface field portion 1722.

In this instance, even though the conductivity types of the emitter part121 a and the back surface field part 172 a are different from eachother, methods for forming the selective emitter structure and theselective back surface field structure are equal to the processes ofFIGS. 5A to 8C or FIGS. 10A to 10C. That is, as described, impurity ionsfor forming the emitter part 121 a injected into the substrate 110 has asecond conductivity type different from that of the substrate 110, butimpurity ions for forming the back surface field part 172 a injectedinto the substrate 110 has a first conductivity type equal to that ofthe substrate 110.

Thus, when the selective back surface field structure is formed in themanner shown in FIGS. 8A to 8C, a back surface field region of the firstconductivity type at the back surface of the substrate 110 is formed,and then an etch prevention layer is selective or partially formed onthe back surface field region to expose portions of the back surfacefield region. Then the exposed portions of the back surface field regionare removed from a surface of the back surface field region by apredetermined thickness and the etch prevention layer is removed. Thus,etched portions of the back surface field region is formed as the firstback surface field portion and the remaining portion (that is, portionsof back surface field region on which the etch prevention layer ispositioned) of the back surface field region is formed as the secondback surface field portion. When the portions of the back surface fieldregion are removed, a damage portion existing at the etched portion ofthe back surface field region is also removed.

When the selective back surface field structure is formed in the mannershown in FIGS. 10A to 10C, a first impurity portion having a firstimpurity doped thickness (concentration) and a second impurity portionhaving a second impurity doped thickness (concentration) greater thanthe first impurity doped thickness (concentration) are formed by using amask 85 equipped an ion implantation equipment or positioned directly onthe substrate 110, or a first impurity portion of the first conductivitytype may be formed at the entire front surface of the substrate 110 bythe ion implantation method, and then impurity ions of the firstconductivity type may be further selectively or partially injected intothe first impurity portion, to form portions of the first impurityportion as a second impurity portion.

Next, a heat treatment is performed on the first and second impurityportions to form a back surface field region of first and second backsurface field regions, and the back surface field region is removed by apredetermined thickness to form the back surface field part 172 aincluding the first and second back surface field portions. Since theback surface field region includes a damage portion at and/or near theback surface field region, the damage portion is removed during removingof the back surface field region.

In an alternative example, the back surface field part of the bifacialsolar cell is in contact with only the back electrode 151 and the backbus bars 152, and in this instance, when the back bus bars 152 areomitted, the back surface field part contacts only the back electrodes151. That is, the back surface field part may be not positioned atportions of the substrate 110 between adjacent back electrodes 152. Inthis instance, a mask is selectively or partially positioned on the backsurface of the substrate 110 and impurity ions of the first conductivitytype are injected into the back surface of the substrate 110 toselectively or partially form an impurity portion of the firstconductivity type on the back surface of the substrate 110. Then, aportion of the impurity portion is formed as the back surface fieldregion by heating the impurity and a portion of the back surface fieldregion is removed from a surface of the back surface field region by apredetermined thickness, to form the back surface field part.

Unlike the solar cell shown in FIG. 1, at least one of the emitter part121 a and the back surface field part 172 a may have the selectiveemitter structure or the selective back surface field structure.

When the back surface field part has the selective back surface fieldstructure formed by the ion implantation process, the activation processand the etching process, the solar cell may obtain a quantum efficiencydepending on a wavelength of light, as shown in (A) and (B) of FIG. 12.

(A) of FIG. 12 shows a simulated graph of an external quantum efficiency(E.Q.E) depending on a variation of the wavelength of light, and in thegraph, the first back surface field portion which is a low impuritydoped portion has an impurity doped concentration of about 3×10¹⁹ cm⁻³and the second back surface field portion which is a high impurity dopedportion has an impurity doped concentration of about 2×10²⁰ cm⁻³.

As shown in (A) of FIG. 12, the maximum wavelength of light absorbedinto the substrate 110 not passing through the substrate 110 is about1200 nm, and at the back surface of the substrate 110 absorbed light ofa wavelength of 900 nm and more, an external quantum efficiency LG1measured at the first back surface field portion, at which a damageportion is removed is larger than an external quantum efficiency HG1measured at the second back surface field portion at the back surface ofthe substrate110, at which a damage portion exists.

(B) of FIG. 12 shows a graph of an internal quantum efficiency (I.Q.E)depending on a variation of a wavelength of light obtained based on theexternal quantum efficiency shown in (A) of FIG. 12. As shown in (B) ofFIG. 12, an internal quantum efficiency LG2 measured at the first backsurface field portion, at which a damage portion is removed is largerthan an internal quantum efficiency HG2 measured at the second backsurface field portion at the back surface of the substrate 110, at whicha damage portion exists.

Although embodiments have been described with reference to a number ofillustrative embodiments thereof, it should be understood that numerousother modifications and embodiments can be devised by those skilled inthe art that will fall within the scope of the principles of thisdisclosure. More particularly, various variations and modifications arepossible in the component parts and/or arrangements of the subjectcombination arrangement within the scope of the disclosure, the drawingsand the appended claims. In addition to variations and modifications inthe component parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

What is claimed is:
 1. A method for manufacturing a solar cell, themethod comprising: injecting first impurity ions at a first surface of asubstrate by using a first ion implantation method to form a firstimpurity region, the substrate having a first conductivity type and thefirst impurity ions having a second conductivity type opposite the firstconductivity type, and the first impurity region having the secondconductivity type; heating the substrate with the first impurity regionto activate the first impurity region to form an emitter region from thefirst impurity region; etching the emitter region from a surface of theemitter region to a predetermined depth to form an emitter part from theemitter region; forming a first electrode on the emitter part to connectto the emitter part and a second electrode on a second surface of thesubstrate, which is opposite the first surface of the substrate toconnect to the second surface of the substrate, injecting secondimpurity ions at a second surface of the substrate by using a second ionimplantation method to form a second impurity region of the firstconductivity type, the second surface being opposite the first surfaceof the substrate; heating the substrate with the second impurity regionto activate the second impurity region to form an surface field regionfrom the second impurity region; and etching the surface field regionfrom a surface of the surface field region to a predetermined depth toform a surface field part from the surface field region, wherein thesecond electrode is connected to the second surface of the substratethrough the surface field part.
 2. The method of claim 1, wherein theheating of the substrate heats the first impurity portion at 800° C. to1100° C. in a nitrogen atmosphere.
 3. The method of claim 1, wherein theetching of the emitter region removes a portion of the emitter regionfrom the surface of the emitter region to a depth of 5 nm to 20 nm. 4.The method of claim 3, wherein the emitter region is etched by anetchant composed of nitric acid HNO₃, hydrofluoric acid HF and purewafer.
 5. The method of claim 1, wherein the heating of the substrateheats the first impurity portion at 800° C. to 1100° C. in an oxygenatmosphere.
 6. The method of claim 5, wherein the etching of the emitterregion removes a portion of the emitter region from the surface of theemitter region to a depth of 20 nm to 35 nm.
 7. The method of claim 6,wherein the emitter region is etched by an etchant composed ofhydrofluoric acid HF and pure wafer.
 8. The method of claim 1, whereinthe emitter part comprises a first emitter portion having a firstimpurity doped thickness and a second emitter portion having a secondimpurity doped thickness greater than the first impurity dopedthickness, and wherein the etching of the emitter region comprises:selectively forming an etch prevention layer on the emitter region toexpose a portion of the emitter region and to cover a remaining portionof the emitter region; and etching the exposed portion of the emitterregion from the surface of the emitter region to the predetermined depthusing the etch prevention layer as a mask; and removing the etchprevention layer, wherein the etched exposed portion of the emitterregion is formed as the first emitter portion and the remaining portionof the emitter region is formed as the second emitter portion.
 9. Themethod of claim 1, wherein the first impurity region comprises a firstimpurity portion having a first impurity doped thickness and a secondimpurity portion having a second impurity doped thickness greater thanthe first impurity doped thickness, and wherein the injecting of thefirst impurity ions forms the first and second impurity portions by useof a mask positioned at the first surface of the substrate and use ofthe first ion implantation method.
 10. The method of claim 9, whereinthe mask comprises a first portion having a first exposing area forforming the first impurity portion and a second portion having a secondexposing area for forming the second impurity portion, the first andsecond exposing areas being areas exposing the substrate in a unit areathereof.
 11. The method of claim 9, further comprising forming the firstimpurity region not having the first and second impurity portions at anentire first surface of the substrate by injecting the first impurityions of the second conductivity type at the entire first surface of thesubstrate without a mask, before forming the first and second impurityportions of the first impurity region, wherein the forming of the firstand second impurity portions of the first impurity region forms thefirst and second impurity portions by use of the mask positioned at thefirst impurity region not having the first and second impurity portionsand use of the first ion implantation method.
 12. The method of claim 1,wherein the surface field part comprises a first surface field portionhaving a first impurity doped thickness and a second surface fieldportion having a second impurity doped thickness greater than the firstimpurity doped thickness, wherein the etching of the surface fieldregion comprises: selectively forming an etch prevention layer on thesurface field region to expose a portion of the surface field region andto cover a remaining portion of the surface field region; etching theexposed portion of the surface field region from the surface of thesurface field region to the predetermined depth using the etchprevention layer as a mask; and removing the etch prevention layer,wherein the etched exposed portion of the surface field region is formedas the first surface field portion and the remaining portion is formedas the second surface field portion.
 13. The method of claim 12, whereinthe second electrode is in contact with the second surface field portionand is connected to the second surface of the substrate through thesecond surface field portion.
 14. The method of claim 13, wherein thefirst and second surfaces of the substrate are light incident surfaceson which light is incident.
 15. The method of claim 1, wherein thesecond impurity region comprises a first impurity portion having a firstimpurity doped thickness and a second impurity portion having a secondimpurity doped thickness greater than the first impurity dopedthickness, wherein the injecting of the second impurity ions forms thefirst and second impurity portions by use of a mask positioned at firstsurface of the substrate and use of the second ion implantation method.16. The method of claim 15, wherein the mask comprises a first portionhaving a first exposing area for forming the first impurity portion anda second portion having a second exposing area for forming the secondimpurity portion, the first and second exposing areas being areasexposing the substrate in a unit area thereof.
 17. The method of claim15, further comprising forming the second impurity region not having thefirst and second impurity portions at an entire second surface of thesubstrate by injecting the second impurity ions of the firstconductivity type at the entire second surface of the substrate withouta mask, before forming the first and second impurity portions of thesecond impurity region, wherein the forming of the first and secondimpurity portions of the second impurity region forms the first andsecond impurity portions by use of the mask positioned at the secondimpurity part not having the first and second impurity portions and useof the second ion implantation method.
 18. The method of claim 15,wherein the second electrode is in contact with the second surface fieldportion and is connected to the second surface of the substrate throughthe second surface field portion.
 19. The method of claim 18, whereinthe first and second surfaces of the substrate are light incidentsurfaces on which light is incident.